1. Field of the Invention
This invention relates generally to the field of semiconductor processing. More particularly, the present invention relates to forming photoresist patterns on an exposure field of a semiconductor wafer.
2. Description of the Related Art
The manufacturing of integrated circuit (IC) chips involves many processes. One of the major processes in manufacturing IC chips is photolithography. Photolithography is a process used to transfer masks containing patterns to the surface of a silicon wafer. In a photolithography process, patterns are transferred from a mask to a light sensitive material called photoresist using a light source to print the patterns onto the surface of the wafer. A chemical or plasma etching is then used to transfer the pattern from the photoresist to the surface of the wafer. Fabrication of IC chips may require a number of photolithography processes depending on the complexity of the circuits in the IC chips.
Today, the dimensions of IC components are becoming increasingly smaller. The smaller device dimensions allow more circuit devices to be provided in an IC chip. Accordingly, the precision and accuracy in performing various processes, and photolithography in particular, are critical in producing properly functioning semiconductor IC devices.
In the photolithography process, the printing of mask patterns onto a silicon wafer is typically performed using a projection aligner and stepper device. Conventional projection aligner and stepper device are described in detail in U.S. patent application Ser. No. 09/141,807, entitled xe2x80x9cAn Apparatus and Method for the Improvement of Illumination Uniformity in Photolithographic Systems, which is incorporated herein by reference. In using a stepper device, for example, an area in a semiconductor wafer exposed to the stepper device is commonly known as a exposure field. The stepper device xe2x80x9cstepsxe2x80x9d over the fields of the surface of a wafer to print mask patterns.
Unfortunately, linewidths of mask patterns often vary across the mask due to nonuniformity in the mask fabrication process. Typically, critical dimensions of a mask are measured for a test feature that is replicated across the mask. Integrated circuit chip customers generally specify both a maximum deviation from a target and a deviation across the mask from the measured average. When used by a customer, the deviation across the mask typically causes linewidth variations across an exposure field, for example, of a stepper device. If a common exposure actinic radiation dose is used for forming a particular photoresist layer on the exposure field of a semiconductor wafer, the critical dimension deviation from a target critical dimension on the mask causes a systematic resist linewidth deviation in the photoresist layer. As can be appreciated, such variation in linewidths may result in IC chips that are either defective or do not perform to application specification.
In view of the foregoing, what is needed is an apparatus and a method for compensating critical dimension deviations across the photomask to improve the yield and performance of IC chips.
Broadly speaking, the present invention fills these needs by providing an apparatus and method for compensating critical dimension deviations across photomask. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, the present invention provides an apparatus for forming a photoresist pattern on an exposure field of a semiconductor wafer. The apparatus includes a light source, a lens, a filter, and a photomask. The light source is adapted to generate actinic radiation for illuminating a photomask pattern onto the exposure field on the semiconductor wafer. The lens focuses the actinic radiation from the light source onto a filter. The filter filters the actinic radiation from the lens. The photomask has a substrate and a layer of reticle. The substrate of the photomask is transparent to the actinic radiation while the layer of reticle defines one or more photoresist patterns. The photomask is partitioned into a plurality of regions and is adapted to attenuate the actinic radiation from the lens in one or more regions to compensate for critical dimension deviations in the one or more regions from the target critical dimension. The plurality of regions in the filter transmits the actinic radiation from the filter to the photomask for illuminating the exposure field on the semiconductor wafer to form a photoresist pattern on the exposure field.
In another embodiment, the,present invention provides a method of compensating for deviations in critical dimensions of photoresist patterns in a photomask. In this method, a photomask is partitioned into a plurality of regions. A critical dimension is then measured for each of the regions in the photomask. Based on the measured critical dimensions, a deviation map is generated to map deviation of the critical dimension from a target dimension for each of the regions in the photomask. From the deviation map, an amount of actinic radiation needed to be attenuated to compensate for the critical dimension deviation from the target dimension is determined for each of the regions of the photomask. Based on the determined attenuation amount of actinic radiation, the transmission of the actinic radiation through each of the regions in the photomask is attenuated such that the critical dimension deviation is compensated to the target dimension for each of the regions in the photomask.
In yet another embodiment, the present invention provides a method for forming a photomask to compensate for deviations in critical dimension of photoresist patterns on the photomask. The method includes: (a) partitioning a photomask into a plurality of regions where the photomask has a substrate transparent to an actinic radiation and a layer of reticle defines one or more photoresist patterns; (b) measuring a critical dimension for each of the regions in the photomask; (c) generating a deviation map indicating deviation of the critical dimension from a target dimension for each of the regions in the photomask; (c) determining an amount of actinic radiation needed to be attenuated to compensate for the critical dimension deviation from the target dimension in each of the regions of the photomask; and (d) adding one or more light attenuating materials to one or more regions of the photomask, wherein the light attenuating materials attenuate transmission of the actinic radiation through each of the regions in the photomask by the determined attenuation amount of actinic radiation such that the critical dimension deviation is compensated to the target dimension for each of the regions in the photomask.
In a preferred embodiment, the actinic radiation is attenuated through the regions in the photomask by implanting or depositing light attenuating materials. When used with a positive resist, the target critical dimension value is the highest critical dimension value selected from the critical dimension map. In this case, the implanted or deposited materials increase the critical dimension of the regions to the target critical dimension. On the other hand, when used with a negative resist, the target critical dimension value is the lowest critical dimension value selected from the critical dimension map. The implanting or deposition of the light attenuating materials, in this case, decreases the critical dimension of the regions to the target critical dimension value.
The implantation or deposition of light attenuating material to selected regions of the mask provides several advantages. For example, the apparatus and method of the present invention provides higher yield in manufacturing integrated circuit chips since the substantially uniform critical dimensions improve device performance and reduce failure. The implantation of light attenuating materials provides further advantages. For example, by implanting absorbing species rather than varying the thickness of the filter layer, the phase of the incoming light is not changed. Thus, the filter does not adversely affect lithography. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.